This invention relates to digital computer network technology. More specifically, it relates to methods and apparatus for implementing dynamic adjustment of modulation profiles for communication channels in an access network.
Broadband access technologies such as cable, fiber optic, and wireless have made rapid progress in recent years. Recently there has been a convergence of voice and data networks which is due in part to US deregulation of the telecommunications industry. In order to stay competitive, companies offering broadband access technologies need to support voice, video, and other high-bandwidth applications over their local access networks. For networks that use a shared access medium to communicate between subscribers and the service provider (e.g., cable networks, wireless networks, etc.), providing reliable high-quality voice/video communication over such networks is not an easy task.
One type of broadband access technology relates to cable modem networks. A cable modem network or “cable plant” employs cable modems, which are an improvement of conventional PC data modems and provide high speed connectivity. Cable modems are therefore instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services. Digital data on upstream and downstream channels of the cable network is carried over radio frequency (“RF”) carrier signals. Cable modems convert digital data to a modulated RF signal for upstream transmission and convert downstream RF signal to digital form. The conversion is done at a subscriber's facility. At a Cable Modem Termination System (“CMTS”), located at a Head End of the cable network, the conversions are reversed. The CMTS converts downstream digital data to a modulated RF signal, which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the digital data is fed to the cable modem (from an associated PC for example), which converts it to a modulated RF signal. Once the CMTS receives the upstream RF signal, it demodulates it and transmits the digital data to an external source.
FIG. 1 is a block diagram of a typical two-way hybrid fiber-coaxial (HFC) cable network system. It shows a Head End 102 (essentially a distribution hub) which can typically service about 40,000 homes. Head End 102 contains a CMTS 104 that is needed when transmitting and receiving data using cable modems. Primary functions of the CMTS include (1) receiving baseband data inputs from external sources 100 and converting the data for transmission over the cable plant (e.g., converting Ethernet or ATM baseband data to data suitable for transmission over the cable system); (2) providing appropriate Media Access Control (MAC) level packet headers for data received by the cable system, and (3) modulating and demodulating the data to and from the cable system.
Head End 102 connects through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108. Each Head End can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the Head End and each distribution node. In addition, because cable modems were not used, the Head End of pre-HFC cable systems did not contain a CMTS. Returning to FIG. 1, each of the fiber nodes 108 is connected by a coaxial cable 110 to two-way amplifiers or duplex filters 112, which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (different frequency ranges are used for upstream and downstream paths). Each fiber node 108 can normally service up to 2000 subscribers. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along with trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a network device 122, such as a subscriber computer.
In order for data to be able to be transmitted effectively over a wide area network such as HFC or other broadband computer networks, a common standard for data transmission is typically adopted by network providers. A commonly used and well known standard for transmission of data or other information over HFC networks is the Data Over Cable System Interface Specification (DOCSIS). The DOCSIS standard has been publicly presented by Cable Television Laboratories, Inc. (Louisville, Colo.), in a document entitled, DOCSIS 1.1 RF Interface Specification (document control number SP-RFIv1.1-I04-000407, Apr. 7, 2000). That document is incorporated herein by reference for all purposes.
Data Communication in Cable Networks
In conventional DOCSIS systems, the CMTS may include a plurality of physically distinct line cards having appropriate hardware for communicating with cable modems in the network. Each line card is typically assigned to a separate DOCSIS domain, which is a collection of downstream and upstream channels for which a single MAC Allocation and Management protocol operates. Typically, each DOCSIS domain includes a single downstream channel and one or more upstream channels. The downstream channel is used by the CMTS to broadcast data to all cable modems (CMs) within that particular domain. Only the CMTS may transmit data on the downstream. In order to allow the cable modems of a particular DOCSIS domain to transmit data to the CMTS, the cable modems share one or more upstream channels within that domain.
Each upstream and downstream channel of the cable network uses a respective modulation profile which is manually configured at the cable Head End by a cable operator or technician. For example, at start-up or initialization of the CMTS, each upstream channel is configured to use a static or fixed modulation profile for receiving communications from the plurality of cable modems using that particular upstream channel. The modulation profile may define a number or parameters to be used by a cable modem when communicating with the CMTS such as, for example, modulation type (e.g. QPSK or QAM), FEC-t byte value (sometimes referred to as FEC strength), preamble, etc. According to conventional techniques, once a particular upstream channel has been configured to utilize a specific modulation profile, the modulation profile for that channel is fixed, and will not change until it is manually re-configured by a cable operator or technician.
The static nature of the modulation profile of each upstream channel becomes problematic, for example, when channel conditions deteriorate. According to conventional techniques, when the channel conditions on a particular upstream channel deteriorate, the modulation profile of the upstream channel must be manually changed in order to compensate for the deteriorating channel conditions. However, by the time the modulation profile of the upstream channel is manually changed, throughput on the upstream channel has already been affected. As a result, cable modems on using the upstream channel may be taken off line by the CMTS.
By way of illustration, it is well known that data can be transmitted much more rapidly using QAM type modulation rather than QPSK type modulation. However, the use of QAM type modulation requires more stringent signal-to-noise ratio (SNR) standards than QPSK type modulation. For example, according to the DOCSIS standard, QAM16 modulation requires a minimum SNR of 25 dB, whereas QPSK modulation only requires a minimum SNR of 15 dB. In conventional HFC networks, it is desirable to use QAM16 modulation on the upstream channels since this modulation type allows for faster data transmission than QPSK. However, as ingress noise increases on an upstream channel using QAM16 modulation, the SNR on that channel begins to decrease. As a result, data being sent from one or more cable modems to the CMTS via the upstream channel may be lost or dropped. As a result, the effective communication rate of the upstream channel is compromised. Moreover, the loss of data on the upstream channel may, in turn, lead to one or more of the cable modems on the upstream channel being taken off-line by the CMTS.
In light of the above, it will be appreciated that there exists a continual need to improve access network configurations (such as, for example, HFC networks) in order to adapt to changing network conditions, and to improve data communication across the access network.